1. Field of the Invention
The present invention relates to a sense amplifier, for instance, which is used for a semiconductor memory having a resistance change element as a memory cell.
2. Description of the Related Art
A magnetic random access memory (MRAM) is one of semiconductor memories having a resistance change element as a memory cell.
As for write of the magnetic random access memory, there are known two systems of a magnetic field write system utilizing a magnetic field generated by a write current, and a spin injection write system utilizing a spin torque by spin-polarized electrons.
The spin injection write system is widely noticed as an effective technique for realizing a large capacity of the magnetic random access memory (for instance, refer to U.S. Pat. No. 5,695,864).
Its characteristic lies in a point that magnetization of a magnetic element of nano scale is directly controlled by the spin-polarized electrons. That is, in the magnetic field write system, a problem of a wrong write of a non-selected cell caused by spread of a magnetic field is generated, while in the spin injection write system, such a problem is not generated. Further, based on a characteristic that the smaller the size of the magnetic element becomes, the smaller the value of the spin injection current necessary for magnetization reversal becomes, the spin injection write system is advantageous for high integration, low power consumption and high performance.
However, in the spin injection write system, since a read current becomes a minute value, it becomes necessary for the spin injection write system to develop technique to sense a minute current difference at high speed.
Specifically, in this system, although the spin injection current is caused to flow into a magnetoresistive element directly, direction of this spin injection current should be changed in accordance with a value of write data. That is, the read current invariably has the same direction as the spin injection current flowing into one direction. Thus, in order not to destroy the data at the time of reading, the read current should be made substantially smaller than the spin injection current.
However, when making the read current small, naturally, the current difference in the read current in accordance with states “1”, “0” of a magnetoresistive element also becomes small. That the current difference becomes small means a time to sense it becomes long. That sensing time becomes long means a time during which the read current is caused to flow continuously becomes long. Such long-time continuous flowing out of the read current increases a current consumption at the time of reading, and enhances a magnetic reversal probability of the magnetoresistive element, thereby generating a problem of data destruction.
Study of a sense amplifier which is to sense a minute current difference at high speed and thus suitable for such a use is performed.
For instance, in Jpn. Pat. Appln. KOKAI Publication No. 2005-285161, there is disclosed a technique that a MOSFET for strengthening a charge of two output nodes of a sense amplifier is turned on at the same time of start of a sense operation. However, in this technique, there is a problem that since the strengthening of the charge is performed before the time point sensing of the minute current difference is performed enough, the minute current difference is erroneously detected. In particular, in the case where there is variations in charging performance between the two output nodes of the sense amplifier, the variations make the minute current difference further small to generate erroneous operation of the sense amplifier.
Further, in Travis N. Blalock et al., “A High-Speed Clamped Bit-Line Current-Mode Sense Amplifier,” IEEE J. Solid State Circuits, April 1991, vol, 26, pp. 542-548, there is disclosed a technique in which the quasi-stable state of the latch is destroyed while converting the minute current difference between the cell current and the reference current into the potential difference by releasing equalizing of two output nodes of the sense amplifier at the same time as start of the sense operation. However, in this technique, there is the problem that since the potential difference generated by the minute current difference is also minute, the sense operation becomes difficult.